Measuring and controlling apparatus



May 29, 1951 J. c. MouzoN 2,554,717

MEASURING AND CONTROLLING APPARATUS INVENTOR. JAMES C. MOUZON BY M ATTORNEY.

May 29, 1951 J. c. MoUzoN 2,554,717

MEASURING CONTROLLING APPARATUS Filed Jan. 26, 1946 y 2 Sheets-Sheet 2 FIG. 2

les

41 F|G.3 l FIG. 4

INVENTOR. JAMES C. MOUZON www ATTO R N EY.

Patented May 29, 1951 MEASURING AND CONTROLLING APPARATUS James C. Mouzon, Philadelphia, Pa., assig'nor, by mesne assignments, to Minneapolis-Honeywell Regulator Colnpany, Minneapolis, Minn., a corporation of Delaware Application `lanuary 26, 1946, Serial No. 643,780

(Cl. S18-28) 3 Claims.

The present invention relates to improvements in apparatus having especial utility in measuru ing and/or controlling the magnitude and changes in magnitude of electrical, thermal, chemical, physical and mechanical quantities or qualities. An object of the invention is to provide improvements in electrical measuring and/or controlling apparatus to the end that there may be obtained increased sensitivity of response and freedom from extraneous and deleterious effects tendingv to interfere with the desired operation of the apparatus.

In sensitivev electrical measuring and/or ccntrolling arrangements Which have been proposed in the prior art, it is customary to employ electronic amplifiers to amplify for measurement and control purposes a small electrical signal derived from and varying in accordance with the change in a variable condition under measurement. Due to the presence of extraneous electrical and magnetic fields, the sensitivity of response of the electronic amplifiers of the prior art arrangements is limited to a value determined by the level ofthe interfering stray voltages picked up in various portions of the amplifiers. Such stray voltages may be introduced at any portion of the amplifier circuit, as for example, the input circuit thereof, or in an advanced stage, or may be introduced into circuits associated with the amplifier but external thereto, for instance the measuring circuit in which are produced the electrical signals or voltages it is desired to amplify. 'If the sensitivity of the amplifier is increased to such an extent that the amplifier is responsive to normal signals which are of the same order of magnitude as the stray voltages picked up, the amplifier is unable to distinguish between the desired and undesired signals and will amplify both, causing the operation of the associated apparatus to be erratic and unstable, and in some cases, unsuitable for control purposes.

According to the present invention, simple and efficient means are provided to maintain the level of the stray signals in the measuring circuit and in the amplifier at an extremely low value, thus permitting increased sensitivity of response of the apparatus Without objectionable interference from stray signals.

Another object of the invention is to provide measuring and controlling apparatus incorporating improved safety provisions to modify the controliupon failure of thecondition responsive means or other components of the apparatus as required either to actuate an alarm indicative of such failure or to provide safe operation.

This feature of my invention is of particular utility when embodied in ypotentiometric temperature measuring and controlling apparatus in which a thermocouple is employed as the primary condition responsive element. Because of their exposure to high temperatures, deleterious atmospheres, and vibrations Within a furnace, thermocouples sometimes burn out or otherwise become open circuited. When the apparatus is arranged to vary the supply of fuel to the furnace as required to maintain the temperature thereof approximately constant, such open circuiting of the thermocouples tends to operate through the apparatus to effect an increase in the supply of fuel irrespective of the value of the furnace temperature, giving rise to a dangerous and otherwise undesirable condition of operation.

In order to avoid such an occurrence, it has been proposed in the prior art to provide means for introducing a normally ineffective signal or voltage of controlled magnitude and phase into the electronic amplier, which signal becomes operative upon thermocouple or potentiometric network open circuiting or failure of other components of the apparatus to cause operation of the apparatus in a safe sense, that is, in the direction resulting in a decrease in the supply of fuel to the furnace. The eiectiveness of an arrangement of this type is seriously limited, however, by stray signals which may be introduced into the amplifier. For example, if the stray signals should happen to be opposite in phase to the introduced controlled signal, the

latter may be cancelled with the result that the apparatus will not fail safe as intended upon thermocouple or other failure. Moreover, upon open circuiting of the thermocouple, or other potentiometric circuit or apparatus component, the level of the stray signals introduced tends to increase appreciably due to 'the more efficient pick-up circuit then provided, further aggrava"- ing the situation.

According to the present invention means are provided for limiting to a suitably low level the amplitude of the stray signals introduced into the amplifier upon open circuiting of the thermocouple or other potentiometric circuit or apparatus component, thus providing positive assurance of the desired operation of the apparatus upon such occurrence.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

1 is a schematic illustration of a practical embodiment of the present invention;

Figs. 1a and 1b show in detail a component of the device illustrated in Fig. 1;

Fig. 2 is a more or less diagrammatic illustration of the amplifier grounding arrangement employed in the arrangement of Fig. 1, and

Figs. 3, 4 and 5 show modifications of the input circuit of the electronic amplifier provided in the arrangement of Fig. 1.

Referring to Fig. 1, there is schematically illustrated a measuring and controlling apparatus in the form of a self-balancing potentiometer controller for measuring, indicating, recording, and controlling the value of a condition, for example, the temperature within a furnace l. The furnace is heated by a burner 2 the supply of fuel to Which is controlled by a pneumatic valve 3.

A thermocouple 4 responsive to the temperature in the furnace I is connected to act in conjunction with a potentiometric network generally designated at 5 and a slide-wire resistance assembly generally designated at 6. As the temperature within the furnace changes, the potentiometer network is correspondingly unbalanced to create a direct current flow in a branch thereof, the amplitude and polarity of said direct current depending respectively upon the extent and direction of the change in the furnace temperature. f

This direct current iiow produced in the potentiometer network is fed through a converter 1a. by means of which it is translated into an alternating voltage of commercial frequency, for example 25 or 60 cycles per second, and of one phase or of opposite phase depending upon the polarity of the direct current now. The alternating voltage so derived is passed into an electronic amplier generally designated at 8, wherein it is amplified by means of a voltage amplifier generally designated at 9. The amplified voltage is then fed into a motor drive circuit generally designated at I, which controls the operation of a reversible drive motor II. This motor II is connected by mechanisms I2 and I3 to the slidewire assembly 6 in such a manner that operation of the motor in response to the amplified quantity derived from the potentiometric unbalance effects an adjustment of the slide-wire 6 in the proper direction and in the required amount to cause the potentiometer circuit to be rebalanced. The motor Ii also positions indicating and recording means I4 for indicating and recording the temperature existing in the furnace I. Recording means I4 is of Well-known type, and is not shown completely here to avoid undue complication of the drawing.

A controller generally designated at I5 is also operated by the motor I I through suitable means. The controller I5 may take the form of a pneumatic controller such as is illustrated in the C. B. Moore Patent 2,125,681, granted July 26, 1938. This controller I5 operates to control the valve 3 as required to maintain the temperature within the furnace I at a desired value.

The potentiometric network 5 comprises three resistances I8, I'I and I8 connected in series in one potentiometric branch and provided for calibration purposes. Connected in parallel with the resistance I'I is the slide-wire assembly 6, and a Calibrating resistance I9, used to adapt the slide-wire 6 to different range values. The slidewire assembly 6 consists of the resistance element 20 and a collector bar 2l, this bar being in continuous contact with a slider 22, the latter also contacting the element 2G. A member 23 carries the slider 22 and causes the latter to be moved over the resistance element 2c and the collector bar 2I in response to movements of the motor Il through the mechanisms I2 and I3.

Also in parallel with the potentiometric branch containing resistances I6, I'I and I8 is another branch containing a battery 24 and a Vernier resistance 25 connected in series. The vernier resistance 25 consists of variable resistances 26 and 2 in series, these being contacted by a slider 28. There is a lost-motion connection between the control knob 29 and the slider 28, operating so that rotation of knob 29 moves at rst only the lower portion of slider 28 which is in engagement with resistance 2l'. Further rotation of the knob 29 causes the upper portion of slider 28 to move over the resistance 26. A coarse and a fine adjustment of the resistance in series with battery 24 is thereby provided.

Connected in parallel with the aforementioned potentiometric branches is another branch comprising a compensating resistance 30 and a standardizing resistance 3l connected in series, and joined at the point 32. The resistance 30 serves to compensate for temperature changes at the cold junction of the thermocouple, and is preferably made of copper or other material having a positive temperature coemcient of resistance. The resistance 3I is provided for standardization purposes. In order to avoid undue complication of the drawing, no standardizing provisions have been illustrated, but it will be understood that manual or automatic standardizing means of known construction may be employed, if desired. With the exception of resistance 30, the resistance of the potentiometer circuit are preferably formed of manganin wire having substantially no temperature coefficient of resistance.

The converter 'Ia includes a vibrator 'I which may be of the type disclosed and claimed in the copending application of Frederick W. Side, Serial No. 421,176, filed December 1, 1941, which issued as Patent No. 2,423,524 on July 8, 1947. As shown in Fig. 1, vibrator 'I includes a reed 33 carrying a pair of contacts 34. The reed 33 is under the influence of a varying magnetic field produced by an electromagnetic assembly 35 and is caused to vibrate between the contacts 36 and 3'I, alternately making contact therewith. Reed 33 is connected to terminal 3B, contact 36 is connected to terminal39, and contact 31 is connected to terminal 40. The electromagnetic assembly 35 consists of a coil 4I and a permanent magnet 42, so arranged that when an alternating current is caused to flow through the coil 4I, an alternating magnetic field is set up which causes the reed 33 to vibrate between contacts 36 and 31 at a frequency of vibration equal to the frequency of the alternating current in coil 4I. Preferably, this alternating current is of commercial frequency, namely 25 or 60 cycles per second. The mounting structure of the coil, magnet, reed, and contacts is grounded through lead 43 and a chassis connection. at the con- A tainer of a multiple section electrolytic condenser 45 to be hereinafter described. The metal can containing the'vibrator is grounded through a chassis connection 4.5.

The converter 1a also includes an input transformer 41 comprising a core 58 on which are Wound a secondary winding 49 and a centertapped primary winding 59, having equal winding sections 5| and 52, which lare joined at the terminal 53. A. shield 54 is placed between the primary and secondary windings, and is grounded through a chassis connection 55. The core 48 and mounting can 56 of the transformer 41 are also grounded through chassis connection 55. The ends of primary winding 5i! are connected to terminals 51 and 53, respectively, and the ends of secondary winding 43 are connected to terminals 59 and 90, respectively.

The thermocouple 4 is connected in a series circuit including the vibrator 1, transformer 41, potentiometric network 5, and slide-wire assembly 6. Specifically, one terminal of the thermocouple 4 is connected by a conductor 6| to the terminal 53 of the transformer 41. The other terminal of the thermocouple is connected by a conductor 62 to one end of a resistance 53, the latter acting in conjunction with a condenser 64 to provide damping, or anti-hunting operation of the motor and elimination of undesired effects due to stray electrical fields. The opposite end of resistance 63 is connected through a conduc-4 tor 65 to the point 32 on the pctentioinetric network 5. The condenser E4 is connected between conductors 6| and 65.

As shown, one end of the slide-wire resistance is connected to terminal B5, to which is also connected conductor 91. The latter is connected to the common junction of resistances i5 and i1. The other end of resistance 29 is connected to terminal 58, to which is also connected conductor 69. The latter is connected to the common junction of resistances I1 and I8. The resistlance I9 is connected between conductors G1 and 69. The collector bar 2| is connected through terminal 19 and conductor 1| to terminal 38 of reed 33 of vibrator 1.

The battery 24 operates to produce a difference of potential between the terminal 15 of the slidewire assembly 6 and the point 32 of the potentiometric network 5. tential difference is determined by the position of slider 22 along slide wire resistance 2e. This potential difference is opposed to the E. M. F. produced by the thermocouple 4. As the slider 22 is moved in a clockwise direction over the resistance 2i), the potentiometrio potential diierence between points 32 and 19 is increased. Similarly, as slider 22 is moved in a counter-clockwise direction, the potentiometric potential difference between points 32 and 13 is decreased. When the potentiometer is balanced, the potentiometric potential difference between points 32 and 1G is equal and opposite to the thermocouple E. M. F. between these points, and therefore, no current flows in the therm'ocouple circuit or through the primary 50 of transformer 41.

For purposes of illustration, and not by way of limitation, it may be stated that the resistance between the points 38 and 53 of the vibrator 4 and the input transformer 41, respectively, is of the order of .200 ohms in a practical working embodiment of the invention.

The voltage ampliiier 9 of electronic amplier 8 comprises three stages of voltage amplication, and aincludes two vacuum tubes 12 and 13, the

The magnitude of this po- 'f vibrator 1, the transformer 41 and the multiple electrolytic condenser 45. The vacuum tubes 12 and "i3 are of the twin-triode type, such as the type TF7, having two sets of triode elements in a single envelope. rIhe tube 12 contains the two sets of triode elements 14 and 15. The triode 14 consists of a plate 15, a grid 11, and a cathode 18 and is in the rst stage of ampliiication The triode 15 consists of a plate 19, a grid 59, and a cathode 8| and is in the second stage of amplification. A common heater 82 heats the cath- @des 18 and 3|. The usual shielding in the tube l2, not shown, is connected to a central projection on the base of the tube, also not shown, and this projection normally is in contact with a central contact 84 on a, tube socket, not shown. The contact 84 is grounded through conductor 43 and the chassis connection 44. An lexternal tube shield, not shown, is placed over the tube l2, this shield being grounded by making contact with the base of the tube, which base is internally connected to the central projection, which in turn is grounded as explained above.

The vacuum tube 13 contains two sets of triode elements 85 and 83. The triode 85 consists of a plate 81, a grid 88, and a cathode 89 and is in the third stage of amplication. The triode 86 consists of a plate 9|), a grid 5|, and a cathode t2, and with the grid and cathode tied together serves as a diode half-wave rectiiier. A common heater 93 heats the cathodes 89 and 92. The internal shielding of the tube, not shown, is connected to a central projection on the base of the tube, also not shown, and this projection is normally in contact with a central contact 94 on a tube socket, not shown. The contact 94 is grounded through a ground bus 95 and a chassis connection 95.

rThe multiple section electrolytic condenser 45 hereinbefore mentioned comprises a condenser cartridge 45 contained in a metal can 91 as shown in Fig. la. The cartridge 45 consists of a continuous condenser cathode foil and a sectional condenser anode foil separated by an electrolytic separator and wound into a cylindrical form as shown in Fig. la. In Fig. lb, the separate portions of anode foil are shown forming with the cathode foil and electrolytic separator a plurality of filter condenser sections 98, 99, and |60, and a cathode by-pass condenser section il. Connection is made to the anodes of the various condenser sections 98, 99, |99, and lili by conductors 98, 99, |89', and lel, respectively. These conductors are in the form of metallic tabs, being portions of the respective anode foils as shown in Fig. lb. In Fig. la, these tabs are shown projecting from the end of the condenser cartridge 45. Connection is made to the common cathode of the condenser 45 by means of a similar tab 91', which projects from the end oi the cartridge 45 and is connected to the can 91, this connection not being shown in Fig. la.

Although the condensers 99, 99, |98, and I9! have a common cathode, as shown in Fig. lo, these condensers are shown in Fig. 1 in the conventional manner as being separate, in order to avoid confusion of the drawing. In Fig. 1, therefore, a conductor 91 is shown, connecting the cathodes of the condensers 9i?, 99, |80, and to ground at a point |92. This point |52 represents the connection between the four mounting lugs, not shown, which serve to secure the condenser can 91 to the chassis, serving also to ground the cathodes of the condensers 98, 89, |00, and 59| through the cathode foil tab 91', the can 9?, the mounting lugs, and the chassis. Two of these lugs also serve as the chassis connections 44 and 95 for other portions of the circuit as described above. This grounding arrangement is more clearly shown in Fig. 2.

In practice it is expedient to have the condensers 98, 99, lll, and ||l| in the same container because of space saving and cost considerations. It has been discovered, however, that deleterious stray signals may be introduced into the first stage of the voltage amplifier 9 if the cathode bias condenser II is located improperly within the container 91 with respect to the iilter condensers 98, 99, and |96. The optimum location of condenser Il has been found to be a position as far removed as possible from the filter condenser |09, as shown in Fig. 1b, since the latter condenser has across it a ripple voltage described more fully hereinafter, which voltage can leak to condenser ||l| and cause a stray signal to appear in the rst voltage amplifying stage. With condenser ||l| in the above optimum location, the tendency for the introduction into the voltage amplifier 9 of stray signals from this source is substantially reduced, due not only to the increase in the resistance of the leakage path between the condensers |90 and IBI caused by the maximum separation, but also to the shielding between these condensers afforded by the intervening anode foils of the condensers 98 and 99. It is noted that in the aforementioned practical embodiment of the invention the value of the condenser |9| is 20 microfarads.

In certain cases manufacturing considerations may make it desirable to interchange the positions of the condensers 98 and |El| in Fig. 1b. The resulting placement of the condenser I! closer to the condenser |99 causes an increase in the magnitude of the stray signal picked up by the condenser |s| from the condenser |99, but in some cases this increase in the stray signal can be tolerated, particularly since the anode foil of the condenser 99 still serves to shield the condenser from the condenser EEO.

The motor drive circuit I9 of electronic amplifier 8 comprises two vacuum tubes |93 and |04, these being of the twin triode type, such as the type '1N7, and used for power arnpiiiication purposes. The tube |93 comprises two triode sections |95 and |96, and the tube |04 comprises two triode sections |91 and |98. rihe triode |65 consists of a plate |99, a grid IU, and a cathode i. The triode 06 consists of a plate H2, a grid ||3, and a cathode ||4. A common heater |55 heats the cathodes and ||4 The internal shielding (not shown) for the tube |93, is connected to a socket contact H4 in the same manner as for the tubes 12 and 13 described above. The contact l I6 is grounded through ground bus 95. The triode section |91 of tube |64 consists of a plate |l1, a grid H8, and a cathode H9. The triode |08 Vconsists of a plate |29, a grid l2 I, and a cathode |22. A common heater |23 heats the cathodes |9 and |22. The internal shield, not shown, for the tube |94 is connected to a socket contact |24 in the same manner as for the tubes 12 and 13 described above. rhe contact l24 is grounded through ground bus 95.

A power transformer generally designated at |25 is used to supply the necessary power to the voltage amplifier 9 and motor drive circuit I9 of electronic ampliner 8. The transformer |25 comprises a core |25 on which is wound a primary winding |21 which is supplied with an alternating current from an A. C. supply, not shown, through conductors |28 and |29, the latter being connected to the supply lines |39 and |3| respectively. A fuse |32 is inserted in the conductor |28 for protecting the transformer from excessive currents. A switch |33 is connected between the conductor |28 and primary winding |21 and provides for the operation of the transformer from two different supply voltages, for example 110 volt and 125 volt supply sources, utilizing the tap |34 on the winding |21 for this purpose. Also wound on the core |26 are three secondary windings |35, |36, and |31, the purpose of which is explained below. A shield |38, connected to the core |25, is placed between the primary and secondary windings, and this shield and the core are grounded through conductor |39 and ground bus 95.

The internal connections of the voltage amplilier 9 are as follows: the terminal 59 of the secondary winding 49 of transformer 41 is connected to grid 11 of the triode 14 by a conductor |44. A resistance |44 and a condenser |42 are connected in parallel between the grid 11 and a ground bus |43, the latter being grounded at the chassis connection S6, to which chassis connection the ground bus and the terminal 69 of the transformer secondary winding 49 are also connected. The condenser |42 is used to stabilize the amplifier, preventing oscillation. The resistance |4| limits the open-circuit impedance of the secondary winding 49 and minimizes the eliects of open-circuit stray signals or voltages,

as hereinafter described. It is noted that in the aforementioned practical embodiment of the invention the value of the resistance |4| is one tenth megohm, and the value of the condenser |42 is 0.015 microfarad.

The cathode 18 of the triode 14 is connected to ground bus |43 through a cathode bias resistance |44. The cathode condenser section |9| of condenser 45 has its anode or positive terminal connected to cathode 18, the cathode of condenser |9| being grounded through the can 91 as explained above. It is noted that in the aforementioned practical embodiment of the invention the value of the resistance |44 is ten thousand ohms.

The plate 1B of the trio-de 14 is coupled to the grid S9 of triode 15 by a. coupling condenser |45. This condenser blocks the D. C. component of the plate circuit voltage on plate 16 from grid 89, but impresses on the latter the A. C. component of the voltage produced across a load resistance |45 connected in the plate circuit of triode 14. A resistance |41 is connected between the grid 89 and ground bus 95. rThis resistance serves as a part of the load for the preceding tube and has developed across it the A. C. voltage to be further amplied. Due to the D. C. IR drop across the resistance |41, the latter assists in providing grid bias for the triode 15. The cathode 8| is connected to ground bus 43. The secondary winding |35 of the transformer |25 supplies energizing voltage to the heater 82 of tube 12 through conductors X and partially shown. Winding |35 also supplies energizing voltage to the other heaters S3, I5 and |23 of the tubes 13, |03 and |94 respectively, as well as to the coil 4| of the vibrator 1. A center tap |43 of winding 35 is grounded through ground bus 55, for the purpose of balancing out nto a large extent the A. C. picked up by the cathodes from the energizing voltage of their respective heaters. The plate 19 of the triode 15 is coupled to the grid 88 of the triode S5 by a coupling condenser |49. This condenser blocks the D. C. component of the plate circuit voltage on plate 19 from grid 88, but

vdenser As explained hereinbefore, such occurrence is grids.

v9 impresses on the latter the A. C. component of the voltage produced across a plate resistance |50, located in the plate circuit of triode 15. A variable grid resistance is connected between the Igrid 88 and ground bus 95, this resistance serving the same purpose for triode 85 as resistance |41 accomplishes for triode 15. Resistance |5| is provided with an adjustable slider for varying the amount of signal impressed on the grid 89 from the plate 19, and thus, comprises a sensitivity adjustment. The cathode 89 of triode 85 is connected to ground bus 95.

The three stages of amplification receive plate voltage from a unidirectional voltage supply circuit which comprises the triode 86, the ilter condensers 98, 99, |80, two filter resistances |52 and |53, and the winding |36 of the transformer |25.

The winding |36 supplies an alternating voltage between the ground bus V96 and the plate 90 of triode 86. The grid 9| and the cathode 92 of the latter are tied together, giving diode operation, and these are connected tothe anode or positive terminal of the rst iilter condenser |00 at the point |54. The condenser |09 is charged by current ow through the diode on alternate half cycles, and produces from this pulsating supply ra relatively constant D. C. voltage. The A. C. ripple component of the D. C. voltage leaving this rst stage of filtering is relatively small.

The rectied current passes into a second and a third stage of ltering consisting of the condenser 99 and the resistance |52 and the condenser 98 and the resistance |53 respectively. In each of the three stages of filtering, the ripple component of the rectied direct current is smoothed out, with the result that there is less ripple at point |55 than at point |154 and still less at point |56. This is necessary since the first stage of amplification can tolerate less ripple in its plate supply voltage than can the second or third stages, due to the high gain in the iirst stage and because of the amplification following the first stage. Similarly, less ripple is allowed .in the plate supply tc the second stage than can be tolerated in the voltage supplied tc the third stage.

It is the ripple component of the plate supply vvoltage appearing across the condenser |00 in the first stage of ltering lwhich tends to leak to the cathode by-pass condenser |0| and consequently tends to cause lastray voltage component to be introduced as a stray signal into the grid-cathode circuit of triode 14. This occurs when the con- |0| is not in its optimum position.

avoided by locating the cathode by-,pass condenser |0| as far away from the filter condenser section |00 as possible inasmuch as the ripple voltage which appears across the latter is of the greatest magnitude in the lter, and hence, is the most troublesome source of stray signals.

The plate 61 is coupled to the grids |0, I3, I8 and |2| of the motor drive tubes |03 and |04 by a coupling condenser |51. The condenser |51 prevents the D. C. component of the Voltage on plate 81 from reaching grids H0, H3, H8 and |2|,

rbut allows the A. C. component of the voltage across a plate load resistance |58 included in the plate circuit of triode 95 to be impressed on said Two resistances |59 and |60 are connected in series between the grids H0, 3, IIB and |2| and atap |6| on the winding |36. A switch |62 is connected between the common junction of the resistances |59 and |60 and the ground bus 95.

This arrangement serves to provide an indication and safe operation of the apparatus in the event the therrnocouple burns out or the thermocouple or potentiometric circuit becomes open circuited in any other manner or in the event of failure of the converter 1a or voltage lamplifier 9. The grid resistances |56 and |60 function in the same manner for their associated tubes as do the grid resistance |41 and |5| for their tubes. When the switch |62 is in the open position, as shown in Fig. l, the grid return to ground for the grids of the motor drive tubes is through the resistances |59 and |60 and a small portion of the secondary winding |36 located between the tap |6| `and the grounded end of. the winding. This permits the introduced A. C. voltage from the portion of winding |36 to be superimposed upon the A. C. signal voltage developed across the resistances |59 and |60 from the plate circuit of triode 85. When the switch |62 is closed, the aforementioned grids of the motor drive tubes are grounded through the resistance |59 and the ground bus 95, the resistance |59 serving as a conventional grid resistance, and no A. C. is supplied to the grids from the portion of winding 36.

Thecathodes I i H4, I9 and |22 of the motor drive tubes |03 and |04 are connected together, and a resistance |63 is connected between these cathodes and the ground lbus 95. The resistance |63 provides the major portion of the grid bias for the rnotor drive tubes |93 and |04.

One end of the secondary winding |31 is connected to the plates |09 and 1 of triodes 35 and |01 respectively. The other end of winding |31 is connected to the plates ||2 and |20 of triodes |06 and |08, respectively. A center tap |64 on the winding |31 is connected by e, conductor |65 to one end of a pair of series connected control windings |66 and |61 of the motor The other end of the pair of control rwindings |66 land |61 is connected to ground bus I by a conductor |68. Thus, it can be seen that the voltage of winding |31 is applied to the motor control windings |66 and |61 through the tubes |03 and |04 and the resistance |63. A condenser |69 is connected between the conductors |65 and |68 and together with the control winding |66 and |61 forms a parallel resonant circuit. This circuit has, therefore, a relatively high external impedance for matchingthe plate impedance of the tubes |03 and |04, and also has a relatively low internal impedance for obtaining desirable motor operation.

The motor has two series connected power windings |10 and |1| which are supplied -with alternating current by conductors |12 and |13 vhas its value so chosen that together with the power windings it forms a series resonant circuit when the motor I! is operating at full speed. Thus, as the speed of motor i increases towards full speed, more current is 'allowed to :flow through the power windings, providing the needed additional torque. This current is in phase -with the line voltage due to the action of condenser |14.

A switch |15 is connected between the A. C. source and the supply lines |36 and |3 A chartdriving motor |16 is connected across lines |36 and 13|.

When the potentiometer circuit is balanced, no current flows through the primary winding 50 of transformer 41. Consequently, no signal is then amplified by the amplifier 9. Under this condition, the triodes 505 and itil and triodes |05 and |08 of the motor drive circuit conduct alter'- nately to equal extents, resulting ina flow of current to the control windings iii and iSi of motor which locks the motor rotor lil', preventing a change in position of slider 22 on resistance 2t. lThe manner in which this result is accomplished is fully described in a copending application of Walter P. Wills, Serial No. 421,173 filed December l, 1941, which issued as Patent No. 2,423,540 on July 8, 1947, wherein the structure producing it is also claimed.

With an increase in temperature in the furnace from a temperature at which the potentiometer is in balance, the potentiometer becomes unbalanced, and a current flows in Ia corresponding direction in the thermocouple and converter circuit. The magnitude of this current is proportional to the amount of the increase in furnace temperature and is interrupted by the vibrator l, and in consequence, pulsating currents flow through Winding 59 of transformer lil to cause the induction of an alternating voltage across the secondary winding 4S. This induced alternating voltage is of the frequency of the voltage supplied to the vibrator coil 4|, and its magnitude is proportional to the amount of unbalance in the potentiometer circuit, and hence, is proportional to the amount of temperature increase which has occurred. For the case under consideration, the voltage produced across the winding i9 may be assumed to be in phase with the A. C. supply voltage.

The voltage across winding 49 is amplified by the voltage amplifier Q, and the amplified quantity controls the motor drive stage I@ so as to cause the latter to deliver to the control windings |68 and Il of mot-or il, a voltage lwhich is in such phase relationship rwith respect to the voltage across the power windings HB and |l| as to cause the required rotation of the rotor Eil needed to move the slider Z2 on the resistance 243 in the proper direction to rebalance the potentiometer circuit. When the potentiometer reaches the new balance point, no current flows in the converter circuit la, the rmotor rotation ceases, and further rotation is prevented as eX- plained above.

Upon a decrease in temperature in the furnace, the converse of the above`v opera-tion occurs, that is, a voltage yappears at the input to the converter 'la which produces across the transformer secondary winding fit an alternating yvoltage proportional in magnitude to the amount of temperature decrease, lbut 180 out of phase with the A. C. supply voltage. As outlined above, this voltage is Iamplified and causes a voltage to be applied across the motor lcontrol windings. Now, however, this yvoltage has such a phase relationship with respect to the power winding voltage that the rotor l'l is caused to rotate in a direction opposite to that caused by a temperature increase. Such rotation then causes movement of the slider 22 in the opposite direction to effect rebalance of the potentiometer circuit.

As was stated hereinbefore, a portion of the winding |36 of the transformer |25 is part of a circuit for providing protection in the event of therrnocouple or other potentiometric ircuit component open circuiting, or other apparatus failure. In this circuit, an alternating current is applied between the grids and the respective cathodes of the motor drive tubes |93 and it@ when the switch |62 is in the open position as shown in Fig. 1. This current is in such a phase relationship with respect to the supply voltage that it tends to cause motor operation in a direction corresponding to that caused by an increase in furnace temperature. The potentiometer slider 22 comes to rest at a point such that the imbalance current in the potentiometer circuit produces an amplified voltage at the motor drive stage which opposes and neutralizes the applied protective voltage. Therefore, in the balanced condition a current of sufficient magnitude to cause the voltage introduced by winding |35 to be cancelled fiows in the thermocouple and converter circuit.

If the thermocouple, potentiometric, or amplider circuit opens at some point, as upon the occurrence of thermocouple burn-out, the normal potentiometer circuit imbalance current is no longer effective to neutralize the voltage applied to the motor drive tube grids by the transformer secondary winding i3. As a result the latter voltage causes motor operation, as explained. Since the motor also drives the fuel regulator l5, the supply of fuel to the furnace will be cut off inasmuch as the motor is caused to drive in the same direction as when a furnace temperature increase occurs. Without this thermocouple or other component open circuit protection, the opening of the thermocouple or other component would not be recognized, and the furnace would be considered falsely to be under control, giving rise to a possibly dangerous, and at any rate undesirable, condition of operation.

When the switch |62 is in the closed position, the voltage from transformer secondary winding |36 is no longer applied to the grids of tubes |93 and iM. This condition is desirable when standardizing the potentiometer.

It is noted that if alternating stray voltages of the same frequency as the alternating volta-ge supply source are introduced into the thermocouple and converter, or potentiometer, circuits when the latter circuits are complete, such voltages are translated by the converter 'la into alternating currents having a frequency twice that of the voltage of the supply source. Such double frequency alternating currents are ineffective to cause operation. of the motor inasmuch as the motor drive circuit il] has a frequency discriminating characteristic and can distinguish between alternating currents of this double frequency and normal alternating currents.

When the thermocouple or other circuit opens, however, as upon the occurrence of thermocouple burnout, or failure of some other apparatus component, stray alternating voltages of the freduency of the supply source which may be introduced into the input to the converter la will not be translated into double frequency, non-motor driving signals in the amplifier 9, but on the other hand, will tend to effect motor operation. Moreover, the magnitude of the stray alternating voltages introduced into the converter or potentiometer circuit upon open circuiting of the thermocouple or other component tends to increase due to the consequent increased impedance of the therniocouple and converter circuit, further aggravating an already undesirable condition of operation.

The introduction of such stray alternating voltages into the thermocouple or potentiometer circuit is particularly undesirable when the apparatus is employed for control purposes as illustrated and described in connection with Fig. 1. Specifically, the open circuit stray voltages may in some instances be of such phase with respect to the supply voltage and of such magnitude that when amplified they will neutralize the protective voltage introduced into the motor drive stage by the transformer secondary winding |3i. Conn seguently, upon open circuiting of the thermocouple, or other apparatus component, the stray alternating voltages tend to deprive the apparatus of its open circuit safe operation.

In order to eliminate such undesirable operation of the apparatus due to the introduction of stray alternating currents into the converter and potentiometer circuits, the resistance 54| mentioned hereinbefore, is connected across the winding 49 of the transformer 41. Resistance I4i limits the impedance of the winding 49 when the primary circuit is opened as upon tnermocouple burn-out or failure of other apparatus component and as a result appreciably reduces the magnitude of the stray voltages which may then be-picked up in the converter circuit, thus obviating the neutralization of the protective voltage introduced into the motor drive circuit by the transformer secondary winding E36. This limiting of the impedance of winding 49 by the resistance |4I when the input circuit to the converter is opened is accompanied by a limiting of the impedance of the input or grid circuit of the triode 14. This renders ineiiective the stray voltages which are picked up in this input or grid circuit, which stray voltages would otherwise cause undesirable operation due to increased impedance of the input circuit occurring as a result of the converter or potentiometer circuit becoming opened. It is noted that in the aforementioned practical embodiment of the invention the impedance of the primary circuit cf the transformer 41 is of the order of 150 ohms when the measuring circuit and converter 'I are connected across the transformer' primary winding and the transformer secondary is connected as shown in Fig. l. Also, the impedance of the secondary circuit of the transformer 41 is of the order of fifty thousand ohms when the transformer is connected as shown in Fig. l, and with the resistance |4| having a value of one tenth megohm as suggested hereinbefore. Under these conditions, the secondary circuit impedance is limited to one tenth megohm upon open-circuit ing of the primary circuit, whereas, this secondary circuit impedance would increase to a value of the order of three tenths niegohm upon open-circuiting of the primary circuit were not the resistance I4! connected in the circuit as shown.

The novel grounding arrangement of the electronic amplifier 8 as disclosed herein serves to keep the level of stray alternating voltages tending to be internally introduced into the amplier at a low, non-objectionable value by preventing ground currents from some portions of the ampliiier from flowing in the input and the first stage of the voltageamplifier 9. The arrangement also eliminates ground loop circuits which by the electromagnetic fields which they set up tend to cause stray signals to be introduced into the amplifier circuit. This novel grounding arrangement of the amplifier circuit, is illustrated schematically in Fig. 1, but the actual physical details of construction are shown in detail in Fig. 2.

In Fig. 2, the container 91 of the condenser 45 is indicated as connected to the chassis by its mounting lugs, shown as conductors, at points 44, 96 and |02. Also shown are the container 56 of the transformer 41, a socket |18 for the vibrator 1, a socket |19 for the tube 12, and a socket contact I 89 on the socket |19 connecting to the cathode 8| of triode 15. The ground bus 43 is grounded at chassis connection 44 and serves to ground the contact 84, mentioned hereinbefore, and the contact |86, both of socket |19, and the contact I8! on the vibrator socket |18, which last mentioned Contact connects to the vibrator frame mentioned hereinbefore.

The ground bus |43 is grounded at the chassis connection 85 and serves as a ground connection for the condenser |42 and resistances |4| and |44. By this arrangement, the ground currents from other components of the amplifier 5 do not flow in the input and rst stage of amplification and are incapable, therefore, of causing any stray voltages therein, as would occur were a common ground bus used for the entire amplier circuit.

A conductor |82 serves as a ground connection for the terminal 5|! of the winding 49 of the transformer 41. This conductor is grounded at the chassis connection 9E. By the use of this separate ground conductor for the secondary winding, the ground currents owing in the other parts of the circuit are prevented from flowing through this Winding, with the result that these currents cause no stray voltages to be developed. in the winding. Were the secondary winding grounded by means of a common ground busl currents fiowing in the ground bus from other portions of the circuit would now through the portion of the bus used as the secondary ground and produce a voltage across this section of the bus. This voltage would then actually be in circuit, with the transformer secondary winding 49 and would be amplified as an input voltage, and thus interfere with the normal desired operation of the apparatus.

The can 5e of the transformer 41 is grounded by means of the chassis connection 55. The core 48 and the shield 54 of transformer 41 are also grounded at the chassis connection. 55 by means of the conductor |83.

The container for the vibrator 'i is grounded at the chassis connection 46 by means of a spring clip. not shown, which is fastened to the chassis and contacts the vibrator when the latter is 'plugged into its socket |18.

The remainder of the ground connections in the circuit of the amplifier' 8 are made to the ground bus 85, which is grounded at the chassis connection 96. All of the circuit components which are grounded to the bus S5 are not shown in Fig. 2 in order to avoid unnecessary complications of the drawing, but the resistances |41 and 5| are shown as examples of the grounding to bus 95.

The power transformer |25 is completely insulated from the chassis on which it is mounted. and its case |84. the core |26, and the shield |38 are grounded by the conductor |39, which connects a transformer mounting screw |85 to the ground bus E5, at the ungroundcd end of said bus. By this grounding and insulating arrangement, ground lcops consisting of the ground bus and the chassis or of the chassis and the transformer case are eliminated. Were such ioops present, they would cause stray voltages to be induced in the circuits of the amplifier.

The undesirable stray signals which have been discussed herein may have their origin in two general places, namely in the measuring circuit and in the amplifier circuit. In Fig. 3 is shown amodied arrangement of the input circuit of the voltage amplifier S of Fig. l effecting a reduction of the stray level of the amplifier by eliminating a large portion of the stray alterhating voltages leaving their origin in the amplier circuit. In this modification, the cathode bias resistance IM, and cathode bias condenser ||i| of Fig. 1 are eliminated, the cathode 'i8 being grounded to ground bus |43, and the triode 'i4 is biased by a grid leak bias resistance |83 and a grid condenser |81. The introduction of some stray alternating voltages in the amplifier circuit is due to leakage between the heater 82 and the cathode 'i8 of the triode i4, which leakage causes a current to flow in the cathode circuit of the triode. This current develops a voltage across the cathode bias resistance and condenser when one is used, which voltage is amplified as a stray signal. The elimination of cathode bias and the cathode bias resistance and condenser, as shown in Fig. 3, eliminates the source of this stray signal.

A modification of the amplifier circuit resulting in a reduction of the stray alternating volt ages picked up in the measuring circuit external to the amplifier is shown in Fig. 4. A pair of variable inductance coils |88 and |89 are inserted in the leads |I and 6|, respectively, to the converter circuit la. By varying the inductance of the coils |83 and |89, different values of stray voltage will be picked up by the coils, these voltages acting to cancel out the stray voltages caused by any unbalance of the hum-bucking windings of the input transformer 41. The coils |88 and |89 may be wound on the same core, the latter being moveable relative to the two coils for permitting the above mentioned inductance adjustments to be made. If desired, the coils |88 and |89 may be wound on separate moveable cores, the inductance of the two coils then being independently adjustable by moving the cores relative to the respective coils. The coils |88 and |89 should be located near the input transformer 4l' so that they will be under the influence of the same stray magnetic fields as the input transformer. This method of stray voltage cancellation results in a marked reduction of the stray level by virtue of the cancellation of stray voltages having their origin in the measuring circuit,

For accomplishing a reduction of the strays in both the measuring and amplifier circuits, the modification shown in Fig. 5 may be employed. In this modified circuit, a variable resistance |93 is placed in the input circuit of the amplier to cause a variable voltage to be introduced into this circuit for cancelling stray voltages present.

`An` alternating potential is placed across the re sistance |90 through the resistances |9| and |92. This potential may have its source in the heater winding of the power transformer. 'Ilhe resistance Si) is grounded to the chassis connection 96 at a center tap |93. It can be seen, therefore, that the resistance |90 provides a means for introducing into the amplier input circuit, by conductor |94 a voltage of such magnitude and phase as will cancel the stray voltages of the input circuit, whether these strays have originated in the measuring circuit or in the input vcircuit itself. This cancellation is possible since the majority of the stray voltages are at supply voltage frequency, which is the frequency of the cancelling voltage, and moreover are either of the same phase as the supply voltageor are of opposite phase since the latter usually is the most troublesome source from which the stray voltages are derived. If desired, suitable phase shifting means may be associated with the resistance |96 for cancelling out stray alternating voltages having different phase angle relationships.

While in accordance with the provisions of the statutes, I have illustrated and described the best forms of the invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

l. A balanceable electrical network including a transformer having a primary winding and a secondary winding one terminal of which is connected directly to ground, means for converting a low-level direct current signal of reversible polarity into an alternating signal of reversible phase in said secondary winding, a source of low-level direct current of reversible polarity connected in series with said converting means and said primary winding, electronic amplifying means having a plurality of amplifying stages to amplify said alternating electromotive force, at least the first amplifying stage having an electric discharge device including an anode. a cathode and a control electrode, a common source of unidirectional voltage to energize the output circuits of all of said amplifying stages, the negative terminal of said source being connected to ground, means to apply the alternating electromctive force produced in said secondary winding between the cathode and control electrode of said first amplifying stage, said last mentioned means being operative to reduce the magnitude of electromotive forces applied between said cathode and control electrode from extraneous sources and including a resistor connected in shunt with said secondary winding and a cathode to ground connection independent of the connection of the other amplifying stages to ground, reversible electromagnetic means, second electronic amplifying means having an input circuit connected to the output circuit of said first mentioned electronic amplifying means and having an output circuit connected to said reversible electromagnetic means for effecting selective actuation thereof, means positioned by said electromagnetic means to balance said network following an unbalance thereof as a resuli'l of variation in said direct current signal, and means to apply an alternating voltage of predetermined magnitude to the input circuit of said second electronic amplifying means to effect actuation of said electromagnetic means in one direction upon failure of the circuit including the transformer primary winding of the first mentioned electronic amplifying means.

2. A balanceable electrical network including a transformer having a primary winding and a secondary winding one terminal of which is connected directly to ground, means for converting a low-level direct current signal of reversible polarity into an alternating signal of reversible phase in said secondary winding, a source of lowlevel direct current of reversible polarity connected in series with said converting means and 17 said primary winding, electronic amplifying means comprising a chassis and having a plurality of amplifying stages to amplify said alternating electromotive force, at least the first amplifying stage having an electric discharge device including an anode, a cathode and a control electrode, a common source of unidirectional voltage to energize the output circuits of all of said amplifying stages, said source comprising a rectifier, a container mounted on but insulated from said chassis and enclosing a power` transformer having a core, a secondary winding connected in circuit with said rectifier and a primary winding adapted to be connected to a source of alternating voltage, a filter comprising several stages of filtering each including a condenser for smoothing out the unidirectional voltage output of said rectifier, a ground bus for advanced amplifying stages connected at one point only to said chassis and interconnecting said chassis, the negative output terminal of said rectifier and the core of said power transformer, means to apply the alternating electromotive force produced across said first mentioned secondary winding between the cathode and control electrode of said first amplifying stage, said last mentioned means including a resistor connected in shunt with said first mentioned secondary Winding and a connection independent of said ground bus from said cathode to said chassis, said connection including a cathode bias resistor and a by-pass condenser connected in shunt therewith, a common container enclosing said cathode by-pass condenser and said filter condensers, said cathode by-pass condenser being electrically shielded from the condenser of the first filter stage, re-

versible electromagnetic means, second electronic amplifying means having an input cirlcuit connected to the output circuit of said rst mentioned electronic amplifying means and having an output circuit connected to said reversible electromagnetic means for effecting selective actuation thereof, means positioned by said electromagnetic means to balance said network following an unbalance thereof as a result of variation in said direct current signal, and means to apply an alternating Voltage of predetermined magnitude to the input circuit of said second electronic amplifying means to effect actuation of said electromagnetic means in one direction upon failure of the circuit including the transformer primary winding of the first mentioned electronic amplifying means.

3. Apparatus as specified in claim 1 wherein said one terminal of said secondary winding and said cathode to ground connection are connected to ground at the same point.

JAMES C. MOUZON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,382,914 Hoxie June 28, 1921 1,953,465 Chesnut Apr. 3, 1934 2,000,677 rllrevor May 7, 1935 2,203,689 Macdonald June 11, 1940 2,376,599 Jones May 22, 1945 2,385,481 Wills Sept. 25, 1945 2,423,534 Upton July 8, 194'? 

